The present invention relates to a method of managing bandwidth for content delivery and related aspects. In particular, but not exclusively, to a method for dynamically determining and/or adjusting a static transmission rate cap imposed on an access link in a communications network to enable a consistent quality of service to be experienced at customer premises equipment when streaming prioritised traffic over access links having differing performance characteristics.
Content delivery systems are well known in the art and provide a range of multi-media formatted data for playback on a device either as streamed content or as files for download and subsequent playback. The content may be broadcast, multi-cast or provided in response to a request for content generated by a playback application running on a device. Live data streams of video and audio information are particularly subject to quality of service issues given the varying bandwidth capacities and error characteristics of the access links to customer premises equipment. Whilst typical examples of content delivered in this way focus on encoded audio and video data, references herein to content or content data may include data enabling fabrication of three-dimensional objects via suitable customer premises equipment, such as 3D fabricators and printers already known in the art.
Currently, content is delivered over a range of network architectures including wireless and fixed or wired infrastructures, such as the public switched telephone network (PSTN). Content providers are currently used to providing content at various levels of encoding to enable content to be delivered in a timely manner to content using devices according to the anticipated transmission conditions and receiver buffer capabilities on the relevant device to which content is being delivered.
In addition to the known differences between wireless networks such as cellular communications networks and wireless local area networks (WLANs) and fixed line (copper and/or optical) connections over the access network to the public switched telephone network (PSTN), there is considerable variation in the capacity and error characteristics of individual lines in the access network itself. Devices which use a Digital Subscriber Line (DSL) broadband service over a copper line and/or optical fibre access network connection receive a great variation in service due to the amount of bandwidth capacity each access line can support, as well as the proportion of traffic that is dropped as a consequence of line errors. This is dependent in part on physical characteristics of each access line connecting the local exchange to the customer premises equipment, for example, the distance to the exchange is a key factor impacting the level of errors in data transmitted on an access line.
Furthermore, some access networks provide prioritisation mechanisms to enable time-sensitive traffic, such as video streaming traffic, to be protected in the event of there being congestion in the network. However, precautions need to be taken to ensure that such traffic does not unnecessarily consume more priority bandwidth than is actually required, as this will unnecessarily disrupt non-priority traffic. One common method for implementing such a precaution is to place a rate cap on the transmission rate used to stream the content. Known systems set a default level of bandwidth as the upper transmission rate limit rather than take into account the actual condition of each access line. For example, a network-wide line rate cap of 95% may be imposed for prioritised traffic streams. In such a network, if a customer's access line supports a maximum bandwidth capacity of 1.4 Mbps, then 95% cap would result in just 1.37 Mbps being available for prioritised traffic. A content distributor of media assets which are encoded, at 1.4 Mbps would therefore not be able to provide a prioritised delivery of such content to that particular customer due to their access line not supporting a high enough maximum bandwidth.
Another issue to consider, however, is a scenario where two customer's different access lines each support a maximum bandwidth capacity of 2 Mbps, however, one access line has historically a higher level of transmission errors. In this case, if the maximum transmission rate used to deliver a prioritised traffic stream on both customer's access lines is set at 1.9 Mbps, this might be sufficient for one to receive a good quality service but the other may not experience such a good quality service as the higher line error rate would result in more errors and a higher need to retransmit lost data. This would mean that the quality of experience provided when streaming the same requested asset to customers would be different on a network-wide basis even though the asset each customer receives has been encoded at the same level and transmitted at the same transmission rate.
Accordingly, in order to maintain a consistent Quality of Experience for prioritised content delivery services to client terminals, some access lines to customer premises over the access network require content to be delivered using more bandwidth than is normally required in order to compensate for higher transmission error rates and consequent re-transmissions.
International Patent Application Number WO 05122500 describes an admission control system which ensures that a minimum percentage of bandwidth is always available for best effort, and which may therefore lead to rejection of new priority session requests to protect existing best effort traffic. However, this does not consider how to manage priority traffic so that it does not affect best effort traffic any more than is absolutely necessary to achieve the desired VoD quality experience.
WO0199342 describes an approach to ATM cell queuing/scheduling wherein various bandwidth parameters are expressed in terms of percentages of available bandwidth as opposed to absolute values—which essentially means that if total available bandwidth drops, then priority queues “lose” bandwidth proportionately, so that lower priorities are not starved.
WO11076737 describes a method for recommending content encoding rates that are appropriate for prevailing network conditions i.e. using lower quality encoding at times when, the network is busy.
WO2011/109101 and WO2009/029202 describe systems in which transmission rates on access links are scaled in response to limitations on the available bandwidth on those links. However, these references do not consider how to deliver a target quality of experience on a given access line in a consistent manner across an access network. Nor is any consideration given as to how to ensure that best effort traffic is not starved of bandwidth any more than it needs to be in a consistent manner across the access network.
The invention seeks to provide a delivery system for media assets encoded at a predetermined constant encoding rate (CBR-encoded content) and which streams requested data to receiving devices as prioritised traffic up to a rate cap that is tailor-made for the access line on which the requested data is being streamed.
FIG. 1 of the accompanying drawings shows a communications system 10 in which a typical content delivery service is provided to customer premises equipment 18a,b,c. As shown in FIG. 1, each CPE 18a,b,c, is connected by a respective access line 20a,b,c to a digital subscriber line access manager (DSLAM) 22 (for example, housed at a local exchange or other distribution point). Authorisation for using a broadband service is provided by a broadband remote access server (BRAS) 24. A suitable application 30 running on each device 18 (see FIG. 2 of the accompanying drawings) is configured to access a service platform 16 with service requests for receiving content. Service platform 16 responds to the content requesting CPE 18a,b,c, with a suitable content address, normally an address provided as a URL to a request router (not shown) for the delivery platform 12 which forwards the request to a server local to the CPE 18 from which the request originates (this enables the communications system to operate in a more bandwidth efficient manner).
The CPE 18 then generates a suitable request, for example, a hypertext transfer protocol (http) request address to the URL provided by the service platform 16. When the delivery platform 12 receives the request, the URL itself may provide information such as the format of encoding to be applied to the content, and this is then used to retrieve an appropriate data item (also referred to herein as a media asset) from data store 14. The delivery platform 12 then streams the requested data over the respective access line 20a,b,c to the respective requesting CPE 18a,b,c. 
If requested data is delivered as a prioritised traffic stream over the access network it enables data to be streamed more reliably to a content using device due to this type of traffic taking priority over other non-prioritised (also known as best-efforts traffic) for any available bandwidth, particularly at times of congestion. FIG. 2 shows an exemplary scenario in which instead of the CPE 18 comprising just one customer premises device configured to use an access line 20, a plurality of devices 28a,b are shown which share an individual access line 20 (for example, access line 20b shown in FIG. 1) using a suitable access point/modem 26. Each device 28a,b as shown is capable of supporting one or more applications 30a, b which generate and/or receive traffic over the access line 20. If more than one device 28, or equally, if more than one application on an individual device 28, require use of the access line 20b at the same time, however, if any bandwidth consumed by a prioritised service to the customer premises is too high, it negatively impacts the performance of any other applications generating non-priority traffic. So, for example, if device 28a is running a web-browser application 30a at the same time as a media player application 30b has requested a live television programme, if the access line they are connected to does not have sufficient available bandwidth, the web-browser may stall as it could be starved of bandwidth for its best-efforts traffic.
Such an exemplary scenario is shown in FIG. 3A for an access line offering an assured rate traffic service set at 1.6 Mbps. If the access line has a 2 Mbps maximum bandwidth capacity, then 0.4 Mbps is available for best efforts traffic, which is not a problem. If, however, an access line is only able to support at most 1.65 Mbps say then only 0.05 Mbps is available to best efforts traffic during time tstart to tend when a traffic stream is being transmitted using the assured rate of 1.6 Mbps. However, if the maximum bandwidth is even lower for that particular access line, the provision of an assured rates may result in no bandwidth at all being effectively available for best efforts traffic during an assured rate transmission (which should result in the requested delivery service being denied).
FIG. 3B shows an alternative bandwidth management scheme for access networks in which prioritised traffic is streamed at up to 95% of the maximum available bandwidth in bursts. This means that there is always at least 5% of bandwidth available for best efforts traffic. For an access line capable of supporting transmission at bandwidths of up to 2 Mbps, this means there will be at least 0.1 Mbps available for best effort traffic, and much more is available if the transmission of prioritised traffic is bursty as there will be intervals between prioritised traffic bursts when the full bandwidth of the line is potentially available for best efforts traffic. However, there are still problems with using a prioritised traffic bandwidth management scheme for managing bandwidth when delivering content to CPEs due to the variation in the maximum line rates each access line can support, as FIGS. 4A and 4B show.
FIG. 4B shows how bandwidth is managed conventionally for an access line with a maximum bandwidth capacity or line rate of 1.6 Mbps, meaning that if a generic line rate cap across the access network of 95% of the maximum available bandwidth is imposed only 0.08 Mbps might be made available for best efforts traffic. FIG. 4A shows the converse situation, where an access line has a high line rate and supports transmission rates up to 30 Mbps. In this case, 28.5 Mbps would be reserved automatically for prioritised traffic using a 95% generic access network cap, leaving 1.5 Mbps for best efforts traffic. Unless content has been encoded for delivery at 28.5 Mbps however, the reservation at this percentile rate of available line capacity results in the bandwidth reserved unnecessarily as more could be made available to best efforts traffic without impacting the delivery of the prioritised traffic stream.
In addition to the limitations mentioned above, several problems are known in the art to be associated with such types of CDN including potential abuse of the priority channel by some media playback clients which consume as much capacity on the access line as possible until their media buffer is full. When this occurs, best effort traffic suffers unnecessarily as more bandwidth is consumed for media playback than is actually required to meet the desired quality of experience.
As mentioned above, it is known in the art for CDN servers to provide support for transmission rate capping. A static unvarying transmission rate cap is imposed on all prioritised traffic streams by the content delivery server. This prevents undue abuse by media playback clients which might otherwise cause too much bandwidth to be consumed by prioritised traffic streams by limiting the maximum transmission rate that can be used by the delivery service when transmitting a media stream to the client. Each media asset is delivered using a predetermined CBR with a transmission rate cap which is typically set slightly above the media encoding rate, for example, at 120% of the encoding rate. This allows the media buffer at the CPE to fill with data and creates a safety margin of content in the buffer so that additional content can be released from the buffer for playback to compensate for transmission errors and subsequent re-transmissions.
The present invention relates primarily to content delivery networks which provide video-on-demand (VoD) services delivered over the access network as a Constant Bit Rate (CBR) media asset to an end-user device on an access line, although this is not the only application of the invention. The CDN supports prioritised delivery of such a media asset to the end-user device to provide delivery assurances regarding the QoE (Quality of Experience) of any playback of the media asset file and is configured so that when there is congestion best effort traffic is dropped to secure bandwidth for services requiring the prioritised traffic. As an example, a typical Quality of Experience metric comprises how many times a video freezes in a given time interval when being viewed during its streamed delivery. Other examples of Quality of Experience measurement parameters are described later herein below.
The invention seeks to obviate and/or mitigate a limitation of known CDNs which affects the delivery of CBR media assets if the communications access network comprises access lines which have different error characteristics. A typical error characteristic may comprise a bit error rate or a packet transmission error. CBR content delivery over some access lines requires slightly higher rate caps than is required on other lines to deliver equivalent quality of experience to CPE for the same CBR media asset. While it is possible to simply use higher rate caps e.g. 98% for all lines, this would be unnecessarily wasteful (i.e. detrimental to best effort traffic) on lines that could work well with lower rate caps.
Aspects of the invention as described herein below and by the accompanying independent claims may be combined with each other and any of the preferred embodiments described herein below and in the accompanying dependent claims, in any suitable manner apparent to one of ordinary skill in the art.
A first aspect of the invention seeks to provide a method for adjusting a transmission rate cap imposed by a service platform on an access link in a communications network comprising a plurality of access links, the method comprising:                receiving a service request for delivery of data from a requesting device, wherein subsequent delivery of said requested data generates prioritised traffic over an access link to the requesting device using a capped transmission rate;        processing the service request to determine one or more performance characteristics of said access link;        determining from said performance characteristics of said access link whether an adjusted transmission rate cap is to be imposed on the access link during delivery of said prioritised traffic to the requesting device, and if so,        generating a source address for said data comprising meta-data indicating said adjusted transmission rate cap to be imposed during delivery of said prioritised traffic over said access link to said requesting device,        and reporting the source address to the requesting device;        wherein the adjusted transmission rate cap imposed on each access link in said communications network enables a measurable quality-of-experience parameter for delivery of said requested data to be data to be of equivalent transmission quality on each said access link.        
The measurable quality-of-experience parameter may enable a measurable contribution to a perceived mean opinion score quality parameter for, delivery of said requested data.
A said perceived mean opinion score quality parameter may comprise a number of degradation in service degradations in a given time interval.
The source address generated may comprise a URL from which requested data can be retrieved by the requesting device.
The meta-data may comprise a pre-fix to the URL for the requested data which indicates a maximum transmission rate to be used when streaming the requested data to the requesting device over said access link.
Another aspect of the invention seeks to provide a service platform for a content delivery network, the service platform being arranged to indicate respective transmission rate caps to be imposed when streaming data to each of a plurality of data requesting devices over respective access lines to which the devices are connected, the service platform being arranged to receive service requests for delivery of data from the requesting devices,                a media selector for processing each service request to determine one or more performance characteristics of said access links, to determine from said performance characteristics of said access links whether an adjusted transmission rate cap is to be imposed on each access link during delivery of prioritised traffic to the respective requesting devices, and to generate source addresses for said data comprising meta-data indicative of said adjusted transmission rate caps,        wherein the adjusted transmission rate cap imposed on each access link in said communications network enables a measurable quality-of-experience parameter for delivery of said requested data to be data to be of equivalent transmission quality on each said access link.        
The source address generated by the above service platform may comprise a URL from which requested data can be retrieved by the requesting device, and wherein said meta-data comprises a pre-fix to the URL for the requested data which indicates a cap on the bandwidth over the access line which can be used when delivering the requested data to the requesting device.
Accordingly, one aspect of the invention seeks to provide a system for varying the amount of rate-capping imposed by a central content delivery platform so that this can be adjusted to take into account content requests from client terminals connected to access lines over which a QoS bandwidth boost is required.
A service platform in such a system first receives a content delivery request from a client terminal and performs a lookup to determine if a QoS bandwidth boost has been applied to the access network line connected to the content requesting client terminal. If it has, it then generates a modified source address for the data, typically in the form of a modified universal resource locator (URL), which is returned to the client terminal. The client terminal then establishes an hypertext transfer protocol (HTTP) request to the modified URL which is hosted by the content distribution platform. The URL provided indicates the level of bandwidth boost indicated for that client terminal's access line, and the content delivery platform can then take this into account when applying a rate cap to the data stream providing the content to the delivery platform so that this does not exceed a suitable level for the relevant access line.